Previously a Conservative, Allen resigned from the party and joined The Independent Group on 20 February 2019. In a joint letter with fellow defectors Anna Soubry and Sarah Wollaston, she described how the leadership had allowed a "hard-line anti-EU awkward squad" to take over the party.[3] She was announced as interim leader of the group, now styled as Change UK, on 29 March 2019.[4]

Allen worked in various corporate positions, including with ExxonMobil and the Royal Mail.[5] In 2008 she joined the family classic-motorcycle paints business, RS Bike Paint Ltd, established by her parents in 1978 and now run by her husband Phil Allen.

Allen has said she was inspired to become active in politics after watching the scenes of the Tottenham riots, and first became a councillor in St Albans. Allen served as a councillor for 18 months before making a bid to become an MP.[6]

In February 2014, Allen ran in an open selection process for the South East Cambridgeshire parliamentary constituency.[7] She was beaten by Lucy Frazer but there was initially a controversy about a possible miscount of votes on Frazer's selection.[8] Frazer was reaffirmed as the candidate in January 2015[9] and in October 2014, Allen was selected as prospective parliamentary candidate (PPC) for South Cambridgeshire.[10] The seat was held by the Conservative Andrew Lansley, then a cabinet minister, who had decided to stand down at the 2015 general election.[11] Allen won the seat in the general election, increasing the Conservative majority and taking 51.1% of the votes cast. The nearest candidate was Labour who took 27.2% of the vote.[2]

In July 2015, Allen was elected to the Work and Pensions Select Committee.[12] Allen made her maiden speech before the House of Commons on 20 October 2015, when she detailed criticism of proposed cuts to tax credits, saying, 'because today I can sit on my hands no longer'. She wanted to criticise the proposed tax credit cuts and to intervene before it was 'too late' to stop the changes to tax credits, even though she did not wish to support the motion tabled by Labour because she disagreed with the party's overall stance,[13][14][15] whilst also not being in favour of the Government's motion over tax credit cuts.[16]Isabel Hardman of The Spectator described her speech as "truly brave" and "well argued".[17] Despite her speech, she voted in favour of tax credit cuts, in line with the Conservative whip.[18][19]

On 5 December 2016, Allen announced her intention to put her name forward for the Conservative nomination for the election of Mayor of Cambridgeshire and Peterborough in May 2017. Allen proposed to combine the role with her current position as MP for South Cambridgeshire.[22] In January 2017, she failed to win the Conservative Party nomination for the role.[23]

In June 2017, Allen was re-elected as Member of Parliament for South Cambridgeshire in the snap general election. Over that summer it was mooted that Jacob Rees-Mogg would be a candidate for the leadership of the Conservative Party. Allen announced that if he became leader she would leave the party.[24]

In June 2018, during a debate on changing the abortion laws in Northern Ireland following a referendum in the Republic of Ireland which would amend the Constitution of Ireland to allow terminations, Allen said that she had an abortion for health reasons when she was younger. She stated: "I was ill when I made the incredibly hard decision to have a termination: I was having seizures every day, I wasn't even able to control my own body, let alone care for a new life... I am a modern, progressive woman in this country and I am proud that this country is my home... How can it be that Northern Ireland will soon be the only part of Great Britain and Ireland where terminations are to all intents and purposes outlawed?"[28]

In September 2018, Allen spoke in favour of a second referendum on the UK leaving the European Union. Allen said she feared the danger to jobs and businesses in her constituency and the whole nation from leaving the EU without a deal. Allen blamed the party's Eurosceptic right-wing for the possibility of a no-deal Brexit, and called them "fiscally and economically irresponsible".[29] Allen said there was "no alternative other than asking – should we come to that, no deal, that looks like that's what's going to happen – then we need to go back to the public to decide what they want us to do next." Allen further said that a second referendum should include the option of staying in the EU under current terms.[30] In early 2019, she co-founded the group Right to Vote.[31]

On 20 February 2019, Allen resigned from the Conservative Party, along with two other MPs from her party (Anna Soubry and Sarah Wollaston), joining the newly formed The Independent Group.[32] On 29 March 2019, it was announced that The Independent Group had applied to become a political party under the name Change UK, and that Allen would be appointed interim leader.[33]